Electron discharge devices employing secondary electron emission



y 1952 G. c. DACEY 3,036,234

ELECTRON DISCHARGE DEVICES EMPLOYING SECONDARY ELECTRON EMISSION Filed Sept. 28. 1959 LIGHT some:

FIG. 3

INVEN TOR G. C. DACEY A TTORNEY States it trite This invention relates to electron systems and, more particularly, to systems in which electron emission is obtained from the surface of a semiconductor body having a PN junction therein as a result of the impingement of energetic particles thereupon.

The copending application of I. A. Burton, Serial No. 532,043, filed September 1, 1955, now Patent No. 2,960,659, assigned to the same assignee of this application, discloses a primary electron emitter comprising a semiconductor body including a suitably biased PN junction therein and having a work-function-lowering coating on a surface close to the junction. In such a structure the emission of electrons into surrounding space is facilitated and a usefully large electron emission results when the junction is biased to the avalanche breakdown point. Such a semiconductor body, therefore, may be used in a variety of electron discharge devices, as is disclosed in the above-identified Burton application.

This invention, on the other hand, is based, to a considerable extent, upon the recognition that a copious and controllable supply of electrons may be provided in the Burton structure at bias voltages in the range immediately below the avalanche breakdown voltage. The presence of these electrons in the thin surface conductivity region provides a ready source of electrons which may be released by the bombardment of electrons or other energetic particles, such as photons. As is known, the electrons emitted secondarily by such bombardment may exceed the number impinging to provide an electron current multiplication. Certain electron devices, such as multipliers and magnetrons, depend, to a considerable extent, upon this phenomenon for their satisfactory operation.

More specifically, it is disclosed in US. Patent 2,790,034 to K. B. McAfee, Jr., issued April 23, 1957, that at a certain reverse bias voltage which is about one-half the avalanche breakdown voltage, carrier multiplication begins to occur which increases until the voltage reaches the avalanche breakdown. Generally, the magnitude of multiplication is in accordance with the empirical relation where V is the body breakdown Voltage of the rectifying junction, V is the voltage applied across the junction and n is a constant dependent on the particular type of junction and the particular semiconductor material.

Thus, in accordance with this invention, as the bias voltage V across the junction is varied in the range below but approaching the breakdown voltage V the number of majority carriers in the thin conductivity-type region contiguous to the emitting surface likewise vary and, since these carriers comprise the source of electrons available for secondary emission, this latter characteristic similarly varies in response to the applied bias.

Accordingly, a broad object of this invention is improved electron systems using secondary electron emis sron.

More specifically, an object of this invention is a non-thermionic cathode system having useful and controllable secondary electron emission.

Other objects are an improved electron multiplier and an improved magnetron.

One exemplary embodiment of this invention comprises a multistage electron multiplier having the general configuration of members, for example, as shown in US. Patent 2,245,605 to J. R. Pierce and W. Shockley, issued June 17, 1941.

In accordance with this invention, each of the cathodes comprises a semiconductor body including a PN junction arranged so that a thin region of N-type conductivity is adjacent to the emissive surfaces. Separate sources of potential are connected across the PN junction of each cathode to provide a bias voltage in the range where multiplication of charge carriers occurs before avalanche breakdown takes place.

Thus, a feature of this inventionis a PN junction semiconductor body having a thin surface conductivity region in which the PN junction is biased in reverse in the region immediately below the avalanche breakdown point. Further, the thin surface region is arranged to receive energetic particles and emit greater numbers of electrons in accordance with the magnitude of the bias voltage applied.

The invention and its further objects and features are explained in the following detailed description taken in connection with the drawing in which:

FIG. 1 is a schematic representation of an electron multiplier in accordance with this invention;

FlGS. 2 and 3 are enlarged views of the cathodes of the device of FIG. 1; and

FIG. 4 is a schematic representation in perspective and partially in section of a multicavity magnetron incorporating the principles of this invention.

Referring now to the drawing, FIG. 1 illustrates in schematic form an electron multiplier of generally conventional arrangement but incorporating in the cathodes thereof an electron emitting portion in accordance with this invention. The electron multiplier 10 comprises an enclosing envelope 11 represented schematically and omitting the details of terminations, such as the usual stem and glass press. The electrode assembly comprises a primary cathode 12, a baffle or focusing electrode 13, an anode or collector electrode 14 and a number of substantially identical auxiliary or secondary cathodes 15 to 15", inclusive.

Although not shown, the electrode assembly is supported by conventional means, for example, by discs and sheets of mica or the like. Likewise, in a conventional manner, the members of the cathode assembly are mounted successively in staggered array along the longi tudinal axis of the envelope 11 so as to provide a continuous path for electrons emitted initially from the primary cathode 12 and secondarily emitted from the cathodes 15 to 15 to the collector electrode or anode 14.

As shown in FIG. 2, the primary cathode comprises two obliquely arranged screen or bafile portions 16 and 17 and a central rectangular portion 18 from which electron emission is obtained. Similarly, referring to FIG. 3, the secondary cathodes IS --15 each comprise a large area screen portion 19, a flange portion 20, and a central rectangular portion 21. The screen or baflle portions 16, 17, 19 and 20 of the primary and secondary cathodes may be formed of a metallic sheet or strip, for example, a strip of silver as in conventional electron multiplier structures.

However, the central portions 18 and 21 of all the cathodes are formed by semiconductor wafers 22 and 23 including PN junctions 24 and 25 disposed parallel and close to the emitting surfaces 18 and 21, respectively. It will be appreciated that certain of the baffle portions, for example portion 17 of the primary cathode, may be also semiconductor wafers including PN junctions if additional emissive surface is desired. 7

Each of the semiconductor elements of the several cathodes contains a single junction. The elements may be of any suitable semiconductive material. It may be remarked that in the case of silicon, the semiconductor elements referred to in this embodiment are advantageously provided with a surface coating of caseiurn, or a like Work-function reducing material, on the electron emitting face in accordance with the teachings of Burton. Semiconductor elements of germanium advantageously are coated similarly on the electron emitting surface. Other semiconductor materials having a higher energy gap coupled with a low work function, such as gallium phosphide, however, are operable even in the absence of such a work-function lowering coating.

Two electrodes are attached to each semiconductor element, one to each conductivity-type region, for applying a bias potential in the reverse direction across the PN junction from the common source indicated by battery 26. Included in one branch of the biasing circuit of each cathode are variable resistance elements 27a to 27 for adjusting the individual applied voltages. In the other branch of the biasing circuit are the fixed resistance elements 28a to 28g arranged to provide that the surface of each successive cathode, starting with the primary 12 to the final secondary cathode is at a slightly more positive potential than the preceding one. A separate variable potential source 29 is provided for ensuring that the collector electrode 14 is at the highest positive potential. The conductors 31 and 32, connected one to the cathode side and the other to the anode side of the device, are connected to a utilization circuit not shown.

The operation of the device of FIG. 1 is similar in its general aspects to that of conventional electron multipliers. Thus, when a light beam from the source 33, which may vary in intensity in accordance with a signal, impinges on the surface of the primary cathode 12, a stream of electrons of corresponding intensity will be emitted from the surface. This stream then is applied without substantial loss of electrons to the secondary cathode 15 and then successively to each secondary cathode in turn until the amplified electron current is collected at the anode 14.

In conventional electron multipliers of the type described in the patent of Pierce and Shockley, the increase in the electron stream at each successive secondary emitter is largely a function of the material, shape and size of the cathodes. However, in the device of FIG. 1 the change in the magnitude of the electron current at each successive cathode is controllable by the alteration of the reverse bias voltage applied across the PN junction of each semiconductor element.

Thus, as set forth hereinbefore, the magnitude of multiplication is a function of the body breakdown voltage V which is fixed for a given device, a constant 12, which for a diffused silicon junction of the so-called graded type is between three and six, and the bias voltage V. Therefore, in the electron multiplier of FIG. 1, the amount of electron current multiplication attained may be adjusted advantageously during the operation of the device by controlling the bias voltages of the individual PN junction elements of the cathodes.

Similarly, another electron device which utilizes secondary emission in accordance with the invention is shown in FIG. 4. In perspective, and partially in section, is shown a portion of a magnetron comprising the anode ring 40 including a series of cavities 41 having a conventional configuration. Centrally disposed within the anode ring 40 is a cylindrical cathode 42 comprising a body of semiconductor material. The cathode 42 has a central portion 43 of P-type conductivity and a thin diffused peripheral region of N-type conductivity '44. Low resistance electrodes 45 and 46 provide electrical connection to the P and N-type regions 43 and 44, respectively. Shown diagrammatically in circuit with electrodes 45 and 46 is a variable potential source comprising the battery 47 and variable resistance 48 for reverse biasing the PN junction of the cathode.

As is well known, the operation of the conventional magnetron depends, to a considerable extent, upon the provision of an electron cloud or stream in the intercathode space as a result of emission from the surface of the cylindrical cathode. In this form of magnetron, the sustaining of the electron stream is dependent, in part, upon electrons secondarily emitted from the cathode surface by the impingement of electrons which move inwardly from the stream and impinge upon the surface. Thus, in the device of FIG. 4, the amount of secondary emission from the cathode surface may be varied within limits by regulating the bias voltage applied, as previously discussed in connection with the device of FIG. 1.

Although not shown, means for initiating operation of the magnetron by primary electron emission may be included in a number of forms. A separate, conventional thermionic cathode of small size may be positioned adjacent the intercathode space or a portion of the semiconductor cathode 43 may be provided with a separate PN junction portion with separate biasing means and having a caesium coating on the N-type surface. Such a portion may be raised to the avalanche breakdown point separately from the remainder of the cathode to induce momentary primary emission in accordance with the teachings of Burton. In another alternative form, the entire cathode momentarily may be raised to the avalanching condition to produce primary emission, particularly from a small caesium coated portion of the cathode surface.

Inasmuch as the above embodiments are merely illustrative, it is to be understood that variations of the proposed structures and applications may be devised by those skilled in the art without departing from the scope and spirit of this invention. In particular, the principles of the invention may find application in any device where secondary emission proves useful.

What is claimed is:

1. In apparatus for producing an electron stream by secondary emission, a semiconductor body including a rectifying junction adjacent to a surface of the body, means for biasing the rectifying junction in the reverse direction in the range just below the avalanche breakdown voltage, means for impinging high energy particles against said surface adjacent to the rectifying junction for inducing electron emission into free space from said body, and means for utilizing said emitted electrons.

2. In apparatus for producing an electron stream by secondary emission, a semiconductor body including a rectifying junction adjacent to a surface of the body, means including a variable voltage source for biasing the rectifying junction in the reverse direction in the range just below the avalanche breakdown voltage, said range extending from about one-half the avalanche breakdown voltage to just less than the avalanche breakdown volt age, means for impinging high energy particles against said surface adjacent to the rectifying junction for inducing electron emission into free space from said body, and means for utilizing said emitted electrons.

3. In apparatus for producing an electron stream by secondary emission, a semiconductor body including a rectifying junction adjacent to a surface of the body, means including a variable voltage source for biasing the rectifying junction in the reverse direction in the range just below the avalanche breakdown voltage, said range being that over which the relation 1 M -1*[ Y- n holds, Where M is the electron multiplication factor, V is the applied voltage across the junction, V is the breakdown voltage and n is a factor having a value between three and six, means for impinging high energy particles against said surface adjacent to the rectifying junction for inducing electron emission into free space from said body, and means for utilizing said emitted electrons.

4. An electron multiplier including a plurality of successive, spaced-apart cathode members, each said cathode member including a semiconductor body having therein a rectifying junction adjacent to a surface of the body and means for biasing the rectifying junction in the reverse direction in the range just below the avalanche breakdown voltage, said surface comprising a portion of the active face of the cathode member whereby electrons impinging on the active faces of said cathode members and adjacent to the rectifying junction of said semiconductor body induce electron emission into free space from said body.

References Cited in the file of this patent Latharn: The Magnetron, Chapman and Hall, Ltd., London, 1952, pp. 103-4. 

